1. Field of the Invention
The present invention relates to a two-optical signal generator for use in an optical fiber link system or the like, and in particular, to two-optical signal generator for generating two optical signals, where a difference between optical frequencies or optical wavelengths of the two optical signals can be adjusted. In the specification, the difference between the optical frequencies is referred to as an optical frequency difference hereinafter, and the difference between the optical wavelength is referred to as an optical wavelength difference hereinafter.
2. Description of the Related Art
An optical fiber link system modulates a digital data signal into an optical signal, transmits a modulated optical signal to a radio base station, performs photoelectric conversion for a received optical signal to output a radio signal, which is then power-amplified and radio-transmitted from an antenna of a radio base station.
FIG. 11 is a block diagram showing a configuration of an optical fiber link system of a prior art.
Referring to FIG. 11, a light source 1 such as a semiconductor laser or the like modulates an optical signal according to an inputted digital data signal, and outputs a modulated optical signal as a first optical signal (optical frequency f1) via an optical combining circuit 3 and an optical branch circuit 4 to an optical amplifier 5. On the other hand, a light source 2 such as a semiconductor laser or the like has its optical frequency controlled by an optical frequency controller 10, and generates and outputs an optical signal as a second optical signal (optical frequency f2) via the optical combining circuit 3 and the optical branch circuit 4 to the optical amplifier 5. In this case, a difference |f1−f2| in the optical frequency is set to, for example, a radio frequency in a millimeter-wave band of several tens to several hundreds GHz, as shown in FIG. 13. The optical amplifier 5 amplifies a power of an inputted optical signal, and then, transmits a power-amplified optical signal to an optical receiver 200 via an optical fiber cable 300 for connecting an optical transmitter 101 and an optical receiver 200 located in the radio base station.
On the other hand, a mixture optical signal obtained by mixing the first and second optical signal branched by the optical branch circuit 4 is photoelectrically converted by a photoelectric converter 6 which comprises a high-speed photodiode with a nonlinear photoelectric conversion characteristic, and then, the photoelectrically converted signal is frequency-converted into a high-frequency signal having a frequency lower than that of the photoelectrically converted signal by a frequency converter which consists of a millimeter wave signal oscillator 7 and a mixer 8. Then, from the components of the thus converted high-frequency signal, a high-frequency signal, which is in proportion to an optical frequency difference |f1−f2| and which has been generated by the above-mentioned nonlinear photoelectric conversion characteristic, is taken out by a band-pass filter 9, and then, is outputted to an optical frequency controller 10. In such an optical frequency loop circuit as configured above, based on the inputted high-frequency signal, the optical frequency controller 10 controls the optical frequency f2 of the second optical signal generated from the light source 2 so that the above-mentioned optical frequency difference |f1−f2| becomes the constant. That is, an interference component between the two optical signals is taken out by the photoelectric converter 6 so that an oscillation frequency difference between the oscillation frequency of the light source 1 and that of the light source 2 becomes a millimeter wave frequency, the taken interference component is compared in frequency with the millimeter-wave frequency of the millimeter-wave signal generator 7, and then, the optical frequency of the light source 2 is controlled in accordance with its error signal. The optical transmitter 101 is disclosed in, for example, a first prior art document of, R. P. Braun, et al., “Optical millimeter-wave generation and transmission experiments for mobile 60 GHz band Communications,” Electronics Letters, Vol. 32, pp. 626-627, 1996 (hereinafter referred to as a first prior art).
In the optical receiver 200, an optical amplifier 11 receives an optical signal through the optical fiber cable 300, and then, outputs the same optical signal to a photoelectric converter 12. The photoelectric converter 12 comprises a high-speed photodiode having a nonlinear photoelectric conversion characteristic, photoelectrically converts the inputted optical signal into an electric signal, and outputs the same electric signal to a band-pass filter 13. From the signal components of the photoelectrically converted signal, the band-pass filter 13 takes out a radio signal of a millimeter-wave band corresponding to the optical frequency difference f0=|f1−f2| which has been generated by the above-mentioned nonlinear photoelectric conversion characteristic, and then, outputs the same radio signal to a radio transmitter 14. The radio transmitter 14 comprises a power amplifier which power-amplify the inputted radio signal, and transmits the same radio signal via an antenna 15 toward, for example, an antenna 91 connected to a radio receiver 210 shown in FIG. 12.
FIG. 12 is a block diagram showing a configuration of the radio receiver 210 according to the first prior art.
Referring to FIG. 12, the radio signal received by the antenna 91 is amplified by a low-noise amplifier 92, which then outputs the received radio signal to a mixer 94 via a band-pass filter 93 which passes therethrough only a radio signal having a frequency f0 of the millimeter-wave band. The mixer 94 mixes the inputted radio signal with a local oscillation signal having a local oscillation frequency equal to an addition result of the above-mentioned millimeter-wave frequency f0 generated by a millimeter-wave signal oscillator 95 to a predetermined intermediate frequency, so as to generate a received base-band signal having an intermediate frequency of a frequency difference between these two signals, and then, outputs the received base-band signal, via a band-pass filter 96 which passes therethrough only the signal component of the intermediate frequency band, and via a signal amplifier 97 to a demodulator (not shown). Then, the demodulator demodulates the received base-band signal into the original digital data signal.
Also, a second prior art document of, D. S. George et al., “Further Observations on the Optical Generation of Millimeter-wave Signals by Master/Slave Laser Side-band Injection Locking,” MWP'97, Post-Deadline Papers, PDP-2, 1997, discloses a constitution of a two-optical signal generator (hereinafter referred to as a second prior art) utilizing a heterodyne interference of two light waves in such a configuration provided with two single-mode semiconductor lasers that an optical signal from a slave laser is intensity-modulated according to a sine-wave signal, and the resultant higher-order mode frequency of the intensity-modulated optical signal is locked into a frequency of a master laser.
Also, the following optical transmission system has been proposed as a system for transmitting optical signals using three distributed feedback semiconductor lasers.
Further, a third prior art document of, Z. Ahmed, et al., “Low phase noise millimeter-wave signal generation using a passively mode-locked monolithic DBR laser injection locked by an optical DSBSC signal,” Electronics Letters, Vol. 31, No. 15, pp. 1254, 1995, discloses a two-optical signal generator (hereinafter referred to as a third prior art) utilizing a heterodyne interference of two light waves, in such a configuration that a distributed Bragg reflection-type semiconductor laser (hereinafter referred to as a DBR laser) having a supersaturated absorption layer is made to oscillate in a plurality of modes, and two side band lights generated using an intensity modulation by an external apparatus is injection-locked into the DBR laser.
A fourth prior art document of, L. Noel et al., “Novel Technique for High-Capacity 60-GHz Fiber-Radio Transmission Systems,” IEEE Transactions on Microwave Theory and Techniques, Vol. 45, No. 8, August 1997, discloses an optical fiber link system (hereinafter referred to as a fourth prior art) for spatial transmission of a millimeter-wave signal. The optical fiber link system comprises:                (a) a millimeter-wave light source for generating two optical signals having a millimeter-wave band frequency difference from each other, by using two, first and second distributed feedback semiconductor lasers; and        (b) a third distributed feedback semiconductor laser having an optical frequency different from those of these lasers of millimeter-wave light sources, where a generated optical signal from the third distributed feedback semiconductor laser is directly modulated according to a data signal. In this fourth prior art, at a transmission apparatus, two optical signals generated by the above-mentioned millimeter-wave light source and an optical signal generated by the above-mentioned third distributed feedback semiconductor laser are wavelength-multiplexed and transmitted. On the other hand, at a receiving apparatus, the former two optical signals and the latter optical signal are wavelength-separated by an optical filter or the like, the respective separated optical signals are photoelectrically converted into electric signals by a photoelectric converter, and then, one of these photoelectrically converted electric signals is mixed with a predetermined local oscillation signal to obtain an original millimeter-wave signal.        
Further, a fifth prior art document of, R. P. Braun et al., “Low-Phase-Noise Millimeter-Wave Generation at 64 GHz and Data Transmission Using Optical Sideband Injection Locking,” IEEE Photonics Technology Letters, Vol. 10, No. 5, pp. 728-730, May 1998, discloses a system (hereinafter referred to as a fifth prior art) having such a configuration that a digital data signal is inputted as a bias current into a first distributed feedback semiconductor laser so as to directly intensity-modulate an optical signal generated by this semiconductor laser according to the sine-wave signal, and then, higher-order modulation components of the resultant optically modulated signal is injection-locked into second and third distributed feedback semiconductor lasers via a 3-dB photo-coupler so as to obtain a two-mode optical signal. In this fifth prior art, when weak modulation is conducted on the second or third distributed feedback semiconductor laser, this leads to an effect of AM-PM conversion due to an action of injection locking and then to such a effect that the optical frequency of the locked output light becomes constant with a phase being modulated. The system of the fifth prior art utilizes this phase modulation and transmits the phase-modulated optical signal.
However, the first prior art suffers from such a problem that the phase noise characteristic of the millimeter-wave signal deteriorates due to a limitation of frequency stabilization by using a frequency control circuit, and this leads to that the optical transmitter of the first prior art cannot be used as it is in radio communications.
Also, the second prior art suffers from such a problem that, although the millimeter-wave frequency can be changed by adjusting the modulation frequency of the sine-wave signal, the setting precision of the frequencies fluctuate with a range up to approximately 200 MHz, and then, it is extremely low.
Further, the third prior art has the distributed feedback optical filter in the laser, so suffers from a small frequency range in which the laser can oscillate, and also from a high Q value as a laser resonator, and this results in not only a small locking pull-in range, but also a small variable range of the carrier wave frequency.
Furthermore, in the fourth prior art, for the purpose of wavelength-separation, it is necessary to provide expensive optical filters for each of the radio base stations at the receiving side, and it is also necessary to provide an additional mixer in a signal processing circuit for processing electric signals. This leads to that it is necessary to provide a lot of electric parts for high frequencies. Therefore, there is such a problem that the cost of the radio base station becomes extremely high as the number of radio base stations increases.
Still further, the fifth prior art has such a feature that it is not necessary to provide any modulation for superimposing a millimeter wave signal onto an optical signal. However, the fifth prior art suffers from such a problem that it is necessary to provide three distributed feedback semiconductor lasers which are well matched in oscillation frequency.